The following disclosure is based on German Patent Application No. 101 24 566.1, filed on May 15, 2001, which is incorporated in this application by reference.
1. Field of the Invention
The invention relates to an optical imaging system having several imaging optical components arranged in succession along an optical axis, means for creating radially polarized light arranged within that region extending up to the last of said imaging optical components, and a crystalline-quartz plate employable on such a system.
2. Description of the Related Art
German laid-open publication DE 195 35 392 A1 discloses an optical imaging system of said type in the form of a microlithographic projection exposure system having, e.g., an i-line mercury discharge lamp as a light source. Said system""s employment of radially polarized light for exposing wafers was intended to improve the coupling of said light into the layer of photoresist, particularly at very large angles of incidence, while simultaneously achieving maximum suppression of any standing waves that might be caused by reflections at the inner and outer interfaces of said photoresist. Various types of radial polarizers that employ birefringent materials were mentioned as prospective means for creating radially polarized light. That radial polarizer chosen was arranged within that region that followed said system""s final phase-correcting or polarizing optical element in the optical train in order that the degree of radial polarization attained prior to incidence on said wafers would remain unchanged. In the event that a catadioptic optical system were employed as said system""s projection lens, the radial polarizer involved should be preferably arranged, e.g., following said optical system""s final deflecting mirror. Otherwise, it might be arranged, e.g., within the preceding illumination system of the projection exposure system.
Radially polarized light, i.e., light that is linearly polarized parallel to its plane of incidence on an interface, is, in general, preferable in cases involving imaging optics, e.g., the imaging optics of microlithographic projection exposure systems, since radially polarized light allows employing highly effective antireflection coatings on their imaging optical components, particularly their lenses, which is a matter of major importance, particularly in the case of microlithographic projection exposure systems with high numerical apertures and at short wavelengths, e.g., wavelengths falling in the UV spectral range, since there are few coating materials that are suitable for use in that spectral range. On the other hand, tangentially polarized light, i.e., light that is linearly polarized orthogonal to the plane of incidence of an imaging light beam on the respective interfaces of the lenses, or similar, involved, should preferably be employed for illumination in order to allow creating the best possible interference-fringe contrasts when imaging objects on, e.g., wafers. In order to allow same, the older German patent application 100 10 131.3 proposed employing a tangentially polarizing element arranged in the vicinity of a pupillary plane of the projection lens, or within the illumination system that precedes same in the optical train that may be assembled from segmented birefringent plates instead of the radial polarizer of German patent disclosure DE 195 35 392 A1.
The invention is based on the technical problem of providing an optical imaging system of the type mentioned at the outset that will both allow comparatively highly antireflective coatings on its optics, which will minimize disturbing reflected stray light, and be capable of yielding an exiting light beam that will allow creating high-contrast interference fringes on an image plane. The invention has a further object of providing a crystalline-quartz plate that can be used in such an optical imaging system.
According to one formulation, the invention solves these and other objects by providing an optical imaging system, in particular a microlithographic projection exposure system, that includes several imaging optical elements arranged one after the other along an optical axis, and a radial polarizer radially polarizing light transiting said optical imaging system and arranged at a location ahead of the final imaging optical element. A polarization rotator that transforms the radially polarized light into tangentially polarized light is arranged at a location following that imaging optical element that follows said radial polarizer in the optical train. The invention additionally addresses these objects by providing a crystalline-quartz plate configured as a polarization rotator, wherein a crystal axis of said plate is at least approximately normal to the plane of said plate.
The optical imaging system according to the invention is characterized therein that it both provides a means for creating radially polarized light with which at least part of the imaging optical components of said system operate, and provides a polarization rotator for rotating the planes of polarization of said radially polarized light and for transforming same into tangentially polarized light in order to yield light that will be tangentially polarized in an imaging plane. Said polarization rotator is arranged following at least one, and preferably several, or even all, of the imaging optical components of said system.
A consequence of said measures according to the invention is that all imaging optical components of said system that are situated between said means for creating radially polarized light and said polarization rotator may operate with radially polarized light, for which they may be highly effectively antireflection coated. In particular, a conventional type of radial polarizer situated at an arbitrary location in the beam path between said light source, i.e., a location ahead of said system""s first imaging optical component, and said system""s final imaging optical component, but ahead of said polarization rotator, may serve as said means for creating radially polarized light. Said polarization rotator will simultaneously transform said radially polarized light, which is preferable for the imaging optical components involved, into tangentially polarized light that will then be incident on said image plane, which will allow creating high-contrast interference fringes thereon. Since said polarization transformation is effected by rotating planes of polarization, the associated intensity losses may be held to low levels.
Under another embodiment of the invention, a plate having an optically active material is employed as said polarization rotator. Optically active materials are known to rotate the planes of polarization of transmitted light, where the angles through which same are rotated will be proportional to the thicknesses of said materials and the constants of proportionality involved will increase as the wavelengths involved decrease. Under another embodiment of the invention, a crystalline-quartz plate serves as said polarization rotator. Although said crystalline-quartz plate will also have birefringent properties, suitably dimensioning and orienting said plate will allow maintaining same at levels so low that the desired polarization rotation will not be significantly altered by the optical activity of said crystalline quartz, at least not in cases involving UV-light, e.g., light having wavelengths of about 157 nm or less.
Under beneficial other embodiments of the invention in which said optical imaging system is a microlithographic projection exposure system, said polarization rotator for rotating the planes of polarization of radially polarized light and transforming same into tangentially polarized light is arranged within a section of said system""s projection lens where the beam path is approximately parallel to its optical axis, in particular, in a pupillary plane, or within a section lying between a pupillary plane and an image plane of same containing, e.g., a wafer to be illuminated. In the case of the first of said arrangements, arranging said polarization rotator in said pupillary plane has the advantage that the approximately normal incidence of light on said polarization rotator yields a high optical activity of same and the effects of off-axis illumination of same, such as birefringence effects, will remain minimal. On the other hand, arranging said polarization rotator closer to said image plane has the advantages that those imaging optical elements situated between said pupillary plane and said polarization rotator will also be penetrated by radially polarized light and that employment of a smaller polarization rotator will be sufficient.
In the case of the crystalline-quartz plate according to the invention, the crystal axis of said plate is oriented approximately parallel to the normal to its surface. A crystalline-quartz plate having said orientation is particularly well-suited to employment as a polarization rotator on optical imaging systems according to the invention.
Under a modified embodiment of the invention, the thickness of said crystalline-quartz plate is 500 xcexcm or less and preferably about 200 xcexcm or less. Plates that thin are particularly suitable for accomplishing the polarization-rotation function on optical imaging systems according to the invention when operated at far-UV wavelengths of 157 nm or less.